Billybob...You provided a thorough sense for the 48RE problem. Welcome to the forums, trust you will find all of us helpful and enthusiastic, we share common interests!
First of all, the 48RE was touted by Chrysler/Dodge Truck as the epitome of the A727-derivative transmissions (though they did not make that connection, the ads were more like "the most rugged O.D. behind the Cummins to date", circa 2005. I've built automatic transmissions professionally since the late 1960s. The A727 was a breakthrough in 1962 but not for 2005.
When this 3-speed platform evolved into the A580, the overdrive, electronic controls, the solenoids and governor sensitivity, the weak band struts, bands and the subassembly, shaft, converter and geartrain weaknesses became the start of a large aftermarket for upgrade parts. This continues with the 48RE. I touched on this with the Sonnax discussion and article at the magazine, and Sonnax is an excellent place to see what rebuilders use to upgrade these transmissions and help them survive, even in normal use and duty.
Is the 48RE worth building? Yes, they can be. The A727 was a bulletproof transmission that held up behind 425 horsepower 426 Hemi V-8s, 440 B-blocks in Class A motorhomes, and Dodge trucks to at least 1-ton capacity. When this transmission grew into an overdrive, the A580 was the earliest stab at it, and frankly, that's not the best unit, although some do build them for muscle car retrofitting to gain an overdrive. The derivative overdrive four-speeds like the 44RE, 46RE and 47RE each have some virtues, although I find their marginalized parts a bit frightening.
Like these earlier versions, and granted, the 48RE is a bit better than the others, to make a 48RE bulletproof, i.e. capable of pulling a 5th wheel toy hauler with a Cummins 5.9L HO engine, does require a laundry list of upgrades. A heavy duty torque converter and input shaft are always on the priority list, the factory converter is marginal at best. For the sake of those interested, here are just a few links full of insights into the "necessary improvements" for the 48RE if you want to use it in the high performance or ongoing trailer pulling realm (same as high performance, though with less harsh shifting when the trailer is not in tow). We can begin with the quick list, several you are already considering:
This last source at Oregon has a parts list of vital upgrades for the 48RE. When my turn comes to rebuild our 48RE in the '05 Ram 3500 4WD Quad Cab SRW truck (bought new and driven to preserve the powertrain, above all!), I will pay close attention to the strategic parts on a list like Oregon Performance Transmission.
That said, since you're not comfortable with rebuilding the unit yourself, you can use the Oregon source as a yardstick for what parts need to be in a rebuild. Also scan the upgrade parts lists at the major shops like BDS and ATS. I would pore over the Sonnax parts, too, every last one that they recommend for keeping a 48RE alive. BDS, Banks and a host of others build the heavy-duty torque converters.
Go for cost-effective but make sure all of the quality upgrades are in place. The phrase "billet replacement" is useful when you see the size of shafts (like the input) and internals. Heavy duty struts, bands and clutch units, heavy duty bushings/bearings, planets and thrusts, there seems to be no end to how an RE Dodge truck transmission can be improved, even the last one, the 48RE!
Cooling is critical, the mantra with automatic transmissions, so shop coolers, too. Warranty is essential, a real "use anywhere", parts and labor assurance that you can be at Timbuktu and get some kind of recourse with the warranty. (This cleaves toward the major builders like BDS and ATS; frankly, others would be universal, local shop service contracts.) Pulling a trailer, you want a transmission that will not fail—ever, ideally—and if it does for any reason whatsoever, you cannot be "stranded" or left to the mercy of a smaller shop with standard replacement parts that would downgrade your expensive rebuild.
All of this scary stuff aside, I have pulled trailers with our '05 Ram 3500 4WD, and hefty ones at times: 1) the car hauler with the XJ Cherokee on board to Moab, 2) a 27' toy hauler to King of the Hammers, and 3) lots of short hauling and moving chores, including a shop full of machinery and a compact tractor, loads in the 8,000# to 10,000# range. That's still not 12,000#, and every pound is an effort for the Dodge Ram overdrive transmissions through the 48RE.
On that note, we're I to do it all again, the NV5600 six-speed manual transmission would likely have been my choice. My wife is not thrilled to drive a lifted truck with oversized tires, though the truck handles really well, and ride and stance are at least as stable as stock, maybe better. It's the size thing.
That said, for me personally, a manual transmission would have been just fine. Perfect? No, they need clutches and clutch upgrades, possibly a rebuild at some point. Compared to the 48RE, though, there would not be the factor of just plain expecting to do a rebuild—inopportunely in your case. (I'd be a bit less surprised; having seen the inside of my 48RE when I did the Sonnax valve body and accumulator piston upgrades, I've dispelled any illusions.) I'm expecting the 48RE and converter to require a major overhaul or upgrading somewhere between 180K-240K miles, even with the utmost of care. The stock profile seems just too marginal.
I respect your approach and would suggest price shopping the best equipment and most refined rebuilding methods, balancing and blueprinting included in the build, a quality valve body upgrade, essentially all of the rebuilding techniques that include the best parts while addressing each of the 48RE's weaknesses. Again, make a checklist of Sonnax, ATS and BDS approaches, each has a great deal of experience with these units.
Please let us know what you decide to do. Also, check out my steering gear and linkage exchange with Megatron, you'll be doing that upgrade before this is all over. As a footnote, I'm very happy with my decision to change axle gearing to 4.56 when 4.10 would have solved the bigger tire diameter issue. The 4.56 gearing with 34.6" (35") diameter tires works nicely and takes a load off the engine and transmission. Slight increase in engine piston travel per mile, but that will prove negligible if the engine goes 490K miles instead of 500K. Actually, the engine might last longer with less strain and decreased need for turbo boost and higher cylinder pressures. I geared for anticipated trailering, a conventional pull travel trailer in the 6,000#-7500# range.
Thanks for joining us, Billybob! Looking forward to our exchanges...